The present invention pertains to electrical equipment and, more particularly to a species of electrical conduit known as intermediate metal conduit, the invention permitting fittings that are presently designed for use with conventional rigid conduit to be employed interchangeably with such intermediate conduit.
Up to now, the National Electrical Code has allowed the use of heavy wall conduit, which is known in the trade as rigid conduit. Such conduit has a wall thickness equal to Schedule 40 Pipe. It should be noted that by Code definitions, such rigid conduit must be furnished with threads which have a taper of three-quarter inch per foot.
More recently, there has been proposed by industry sources, for inclusion in the 1975 National Electrical Code, a new type of conduit known as intermediate metal conduit (IMC).
A proposal for such conduit is especially timely in view of the recent experiences in this country with shortages of material, such shortages having severely affected the manufacture of rigid metal conduit. The newly proposed intermediate metal conduit uses approximately sixty percent (60%) of the steel since it has a wall thickness of approximately sixty percent (60%) of the rigid conduit wall thickness. Moreover, generally speaking, it is much more easily manufactured.
The present National Electrical Code proposal will allow intermediate grade conduit to be furnished threaded or unthreaded; however, if it is furnished threaded, it is required by such proposal to have a taper in its thread of three-eighth inch per foot. The reason behind the lower requirement on the taper is that in accordance with previous proposals for forming intermediate metal conduit, it could not have a taper of three-quarter inch per foot since the established wall thickness would be insufficient to allow for this latter angle of taper.
Accordingly, it will be appreciated that a source of great difficulty arises in that the threads of the proposed intermediate metal conduit would be designed with a thread system that is not interchangeable with the thread system of rigid conduit. As a result of the aforesaid incompatibility between the thread systems, the ordinary course would be to utilize adapters so as to interface with the conventional rigid conduit fitting and boxes; that is to say, the adapters would be provided with a specially modified female thread on one end and a standard male, rigid conduit, thread on the other end. Otherwise, entirely new lines of fittings would have to be designed for use with the intermediate metal conduit. The latter course would obviously impose a tremendous financial burden on the electrical industry, since the cost of tooling of such a new line of fittings and the additional inventories involved would be extremely costly.
It must be recognized, however, that if adapters are chosen as the expedient to be utilized, they may not always be used. Experience teaches that when other alternatives are available to the worker in the field, he will adopt such alternatives. For example, where the only tool available to him is a conventional threading die and he must thread the conduit at the site, he will use such conventional threading die, but only for a portion of a distance he would normally use it on rigid conduit. By threading only a portion of the normal distance involved, he would thereby avoid the problem of cutting through the conduit. However, such an arrangement is entirely unsatisfactory because, since one would have to shorten the distance as much as four to five thread lengths, the pitch diameter at the end of the conduit would not then permit entry of a hub or other fitting onto the conduit.
In addition to the foregoing, the threading problems as noted above produce a high degree of confusion inasmuch as the different threads, such as the three-eighth inch per foot taper proposed for IMC versus the three-quarter inch per foot for rigid conduit, result in difficulties for workers in using lock nuts and threaded fittings which are not really compatible. In other words, unless the fittings and lock nuts are kept entirely separate, the difference in the taper, which is not readily apparent, would result in incorrect fittings being used. Consequently, a serious safety problem would arise inasmuch as this kind of conduit is often used as a grounding conductor.
Another factor that must be understood is that the present NEC proposal requires that intermediate metal conduit have the same nominal outside diameter as rigid conduit in order that certain fittings which enable threadless connection of rigid conduit be likewise adapted for threadless connection of intermediate metal conduit without modification.
Accordingly, it is a primary object of the present invention to overcome the aforesaid problems and difficulties with respect to achieving compatibility of thread design and to afford a solution thereto.
A more specific object is to ensure that the fittings designed for threaded rigid conduit can also be employed when intermediate metal conduit is used; in other words, that the basic fittings are interchangeable with either kind of conduit.
Another specific object is to provide compatibility of the thread system such that a conventional threading die, typically used at a construction site, can be used without giving rise to safety problems.
A recent proposal which has been described in the literature and which has been directed to overcoming partially the aforenoted difficulties is a technique which is known as a swaging operation and involves forcing a tapered ring die over the end of the conduit. Thus, an initial taper is applied to the intermediate metal conduit prior to the formation of the tapered thread. The difficulty here is that performing the swaging operation in the field involves holding the conduit and delivering the required high degree of force without the use of power equipment.
Accordingly, it is another object of the present invention to avoid the difficulties entailed with a swaging operation at the construction site and to provide for conventional use of a standard threading die at such site.
The principal feature of the present invention resides in the recognition that intermediate metal conduit can be made compatible with the fittings normally used on rigid conduit by fabricating or forming the intermediate metal conduit so as to have an outside diameter that favors the lower side of the wide tolerance range applicable to the outside diameter of rigid conduit. For example, taking the trade or nominal one-half inch size for rigid conduit, such conduit is manufactured with a tolerance such that it will have an outside diameter which varies between 0.856 maximum and 0.809 minimum. Thus, if the intermediate metal conduit is formed so as to have a nominal outside diameter with a dimension of the order of 0.815.+-. 0.005 inches or, in other words, 0.820 maximum to 0.810 minimum, it could then have an E.sub.0 (pitch diameter at beginning of external thread) with a nominal dimension of 0.762. This arrangement permits interchangeability of a rigid conduit fitting such as a hub, since such a hub will be threadable onto an intermediate metal conduit having such a nominal pitch diameter.
It will be apparent as the description proceeds that although only a single example of a one-half inch size has been indicated above, the basic principle of the present invention can be applied as well to a variety of trade sizes, such as the sizes from one-half to four inches, or any other sizes. In other words, the same concept of favoring the minimum side of the rigid conduit tolerances can be followed in all such cases.
Thus, the present invention may be characterized briefly as a method of producing or fabricating intermediate metal conduit by (1) particularly dimensioning its nominal outside diameter such that it falls at the lower end of the wide tolerance range on the outside diameter for conventional rigid conduit; (2) appropriately dimensioning the wall thickness for the IMC.
It should be appreciated that such dimensioning does not affect conduit fill, since the inside diameter of the proposed IMC is still much larger than the inside diameter of the comparable rigid conduit.
Other and further objects, advantages and features of the present invention will be understood by reference to the following specification in conjunction with the annexed drawings, wherein like parts have been given like numbers.